Magento’s constructor injection pattern enables you to flexibly manage your class dependencies. However, constructor injection also means that a chain reaction of object instantiation is often the result when you create an object. (The original object has dependencies that have dependencies, and those objects have dependencies, and so on.)

If a class’s constructor is particularly resource-intensive, this can lead to unnecessary performance impact when another class depends on it, if the expensive object does not end up being needed during a particular request. (You can display a dependency graph of such objects by enabling profiling.)

As an example, consider the following two classes:

class SlowLoading
    public function __construct()
        // ... Do something resource intensive

    public function getValue()
        return 'SlowLoading value';

class FastLoading
    protected $slowLoading;

    public function __construct(
        SlowLoading $slowLoading
        $this->slowLoading = slowLoading;

    public function getFastValue()
        return 'FastLoading value';

    public function getSlowValue()
        return $this->slowLoading->getValue();

Assume that class SlowLoading has a non-trivial performance impact when instantiated (perhaps due to a complex database query or a call to a third-party web API). Because of the dependency injection in the constructor of FastLoading, this impact is incurred if FastLoading is instantiated. Note, however, that the SlowLoading instance is used only in the method getSlowValue, meaning that the resource cost is unnecessary if this method is never called on the FastLoading object.

Proxies are generated code

Magento has a solution for this situation: proxies. Proxies extend other classes to become lazy-loaded versions of them. That is, a real instance of the class a proxy extends created only after one of the class’s methods is actually called. A proxy implements the same interface as the original class and so can be used as a dependency anywhere the original class can. Unlike its parent, a proxy has only once dependency: the object manager.

Proxies are generated code and therefore do not need to be manually written. (See Code generation for more information.) Simply reference a class in the form \Original\Class\Name\Proxy, and the class is generated if it does not exist.

Using the preceding example, a proxy can be passed into the constructor arguments instead of the original class, using DI configuration as follows:

<type name="FastLoading">
        <argument name="slowLoading" xsi:type="object">SlowLoading\Proxy</argument>

With the proxy used in place of SlowLoading, the SlowLoading class will not be instantiated—and therefore, the resource intensive constructor operations not performed—until the SlowLoading object is used (that is, if the getSlowValue method is called).

Because DI configuration is used to inject a proxy, proxies can be dropped in to replace their corresponding classes - or proxy replacements removed - without touching application code.

As a practical example of a proxy, you can see the StoreManager class and then see the generated StoreManager proxy class.